The present invention relates to a spark plug and a manufacturing method therefor, which spark plug is mounted on internal combustion engines, such as automobiles, cogeneration systems, gas-forcedly-feeding pumps, and in particular, to a spark plug equipped with an earth electrode on which a noble metal chip is welded at its tip end facing a center electrode of the spark plug and a manufacturing method for such a spark plug.
A spark plug of this kind is normally used in environments with severe thermal load. Temperature at its earth electrode is therefore easier to increase, frequently resulting in that the temperature exceeding a given temperature. To overcome this drawback, a countermeasure to decrease the temperature of the earth electrode is proposed. One countermeasure is that the earth electrode is embedded in a material having higher heat conductivity (for example, Cu or Ag), and the material is encapsulated into a cladding member made of metal having corrosion resistance and oxidation resistance (for example, a Ni-based alloy).
The greater amount of the good heat conductive material encapsulated in the cladding member, the higher an effect of decreasing the temperature of the earth electrode. Additionally, the electrode temperature becomes higher as being close to a front end of the earth electrode straight facing to the center electrode. Hence, to gain a large effect of decreasing the temperature, it is better to place a good heat conductive material at the tip of the earth electrode. However, if a good heat conductive material of which the melting point is low, such as Cu or Ag, is encapsulated and extended up to a root portion of a noble metal chip secured on the tip of the earth electrode, the material heats up and melts when the chip is welded onto the tip, thereby causing the earth electrode to deform due to buckling.
A Japanese Patent Laid-open publication NO.4-366581 discloses a structure to suppress deformation due to buckling occurring at the earth electrode when welding the chip. Practically, in the structure thus disclosed, a distance between a welded bottom surface of the chip and an end of the good heat conductive material is kept to 0 to 2 mm so that the material is avoided from being positioned immediately under the chip within the earth electrode.
However, the structure disclosed in the above publication sacrifices the temperature decreasing effect at a relatively large proportion, because the good heat conductive material is separated from being positioned immediately under the chip within the earth electrode.
Therefore, an object of the present invention is to provide a spark plug for internal combustion engines, which has an earth electrode, to a tip of which electrode facing to its center electrode a noble metal chip is welded, in which it is possible to not only prevent deformation due to buckling of the earth electrode when welding the noble metal chip but also securing an excellent effect of decreasing temperature at the earth electrode.
In order to realize the above object, one aspect of the present invention is provided by a spark plug for internal combustion engines, which is provided with an earth electrode (40) having a front end facing to a center electrode (30) of the spark plug, a noble metal chip (60) being welded to the front end via a melted portion (61 and 62) formed therebetween. The earth electrode (40) has a cladding member (43) made of metal having corrosion resistance and oxidation resistance, a first core member (44) not only made of metal higher in heat conductivity than the cladding member (43) but also contained in the cladding member, and a second core member (45) not only made of metal higher in melting point than the first core member (44) and superior in heat conductivity than the cladding member (43) but also contained in the first core member (44). The second core member (45) is protrudes, within the earth electrode (40), from a tip of the first core member (44) to a side of the front end of the earth electrode (40).
Further, a tip of a protruding portion of the second core member (45) is located at a position satisfying a distance L1 of not less than 0.3 mm and not more than a distance L0, the distance L1 being defined by a distance between the tip of the protruding portion of the second core member (45) and the front end of the earth electrode (40) and the distance L0 being defined by a distance between a farthest located edge of a melted portion (61 or 62) including the noble metal chip (60) from the front end of the earth electrode (40) and the front end of the earth electrode (40).
Conventionally, metal, such as Cu that is excellent in heat conductivity, it was hard to locate the metal under the chip. By contrast, the spark plug of the present invention adopts, as core members to gain heat conductivity, a configuration in which the first core member having a lower melting point is surrounded by the second core member having a higher melting point. Additionally, owing to the fact that the distance L1 is not more than the distance L0, the protruding portion of the second core member, which is protrudes from the first core member, can be located under the chip.
Further, the distance L1 is set to an amount of not less than 0.3 mm. If the distance L1 is less than 0.3 mm, the second core member is too close to the earth electrode. Thus, setting the distance L1 in that way prevents the second core member from being exposed from the cladding member when the spark plug is in operation.
According to the present invention, heat conductivity can be secured thanks to the first and second core members. In parallel, it is possible to employ, as the second core material, a material having a melting point that is high enough to be freed from deformations during welding of the chip. Hence, to prevent buckling of the earth electrode during an operation of welding the chip and to obtain an effect of decreasing temperature at the earth electrode are both relaxed.
In the foregoing spark plug, the metal constituting the cladding member (43) can be made of either a Ni alloy or a Fe alloy. The metal constituting the first core member (44) can be made of either Cu or a Cu alloy. Moreover, the metal constituting the second core member (45) can be made of a Ni alloy containing nickel of not less than 90 wt %, more preferably, pure Ni.
Still preferably, in the above construction, when taking a diameter of the noble metal chip (60) as D and taking a distance between the tip of the first core member (44) and the front end of the earth electrode (40) as L2, the tip of the first core member (44) is located in such a manner that the distance L2 is larger than the distance L0 in cases the diameter D is larger than 2.2 mm.
According to the present inventors"" verification, it was found that whether or not buckling of the first core member located under the chip occurs depends on the diameter of the noble metal chip, because degrees of heating in welding the chip are changed. A further verification conducted by the present inventors revealed that determining a position of the tip of the first core member depending on the diameter D of the noble metal chip as described above led to more effective advantages than those obtained by the foregoing basic construction.
According to other aspect of the present invention, there can be provided methods for manufacturing spark plugs having the foregoing constructions in a secure manner.
The references enclosed in parentheses in the above constructions correspond to constituents detailed in the following embodiments, but it is not meant that those references limit the scope of the present invention.